It is standard practice, as for example known from German Pat. No. 2,535,989, for an extruder to emit a strand formed of a mixture of wood chips and similar cellulosic plant byproducts and a suitable normally synthetic-resin binder into a shaping passage constituted by a plurality of longitudinally extending and transversely movable walls. Normally these walls are heated and are urged transversely inwardly by appropriate biasing elements, such as hydraulic or pneumatic cylinders, to compact the strand to the desired density as it cures.
In such systems the transverse biasing pressure, which is of course identical to the transverse force the passage walls exert on the strand, is normally decreased when the extrusion pressure drops below a predetermined level. In this manner friction between the strand and the passage walls is reduced and the strand can move longitudinally in the passage. Since the strand is normally extruded pulse-wise, that is in discrete advance steps between which the strand is stationary, the pressure is also relieved pulse-wise. The problem with this operational method is that the density of the finished cured strand frequently varies too greatly; some portions of the strand are insufficiently compacted and others are too hard.